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Max Ze values from DNO

Hi, long time no posting, busy busy I'm afraid.

Concerning supply Ze which is usually quoted as 0.35 max for TNCS, 0.8 for TNS.

On unmetered highway supplies UKPN says

Makes perfect sense as the fuse size (time and Ia)  dictates the max Ze. 

So where did the .35 and 0.8 come from, or go to in this case.

Kind regards

Marc

Parents
  • The values 0.35 ohms and 0.80 ohms are for run-of-the-mill metered supplies from approx 40 A to 100 A. The values were quoted in Engineering Recommendation P23/1, which was updated back in 2018 and P23/2 says something different.

    This article provides a little more information regarding the replacement of the old version of P23/1, from which the rule of thumb values came, with P23/2: electrical.theiet.org/.../

    Unmetered supplies ( larger metered single-phase supplies) are (and to be honest always were) "out of scope" for the rule of thumb.

Reply
  • The values 0.35 ohms and 0.80 ohms are for run-of-the-mill metered supplies from approx 40 A to 100 A. The values were quoted in Engineering Recommendation P23/1, which was updated back in 2018 and P23/2 says something different.

    This article provides a little more information regarding the replacement of the old version of P23/1, from which the rule of thumb values came, with P23/2: electrical.theiet.org/.../

    Unmetered supplies ( larger metered single-phase supplies) are (and to be honest always were) "out of scope" for the rule of thumb.

Children
  • Graham, thank you. I thought that Table 2 was particularly interesting. Does it matter if Ze is say, 1.0 Ω? Well yes if R1 + R2 > 0.37 Ω and your sockets are on a 32 A type B MCB. (Which is how I discovered my "excessive" Ze at home.) If I understand the article correctly, there is no hard and fast limit, just a distribution of values. But of course, I may have misunderstood!